Fluid-Structure Interaction, Pair

) multiphysics node provides a coupling between a pair of boundaries, where one is the boundary of a fluid domain and the other is part of solid structure. The solid material can be modeled either in a neighboring domain or on the boundary itself. In the former case, the Solid Mechanics or Multibody Dynamics interface is used; in the latter, one of the Shell or Membrane interfaces is used.

You need to use this type of coupling if the geometry sequence is set up to form an assembly. In the more common case of a union when the boundary is shared, use the Fluid-Structure Interaction multiphysics coupling.

The effect of the coupling depends on whether a is active in the fluid domain or not. For cases when the structural deformations are so small that the change in the geometry of the fluid can be ignored, you do not have to use a deforming domain. This is called a fixed geometry. Domains with a fixed geometry have fewer degrees of freedom and are less nonlinear and are thus computationally less expensive

	If Deforming Domain is active in any domain, the Include geometric nonlinearity option is automatically selected in all applicable study nodes.

The is the multiphysics coupling feature name. The default (for the first multiphysics coupling feature in the model) is Fluid-Structure Interaction, Pair 1.

The is used primarily as a scope prefix for variables defined by the coupling node. Refer to such variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different coupling nodes or physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the field. The first character must be a letter.

The default (for the first multiphysics coupling feature in the model) is fsip1.

The boundary selection cannot be edited manually. The contents are obtained from the .

This section defines the physics interfaces involved in the Fluid-Structure Interaction coupling. The and lists include all applicable physics interfaces.

The default values depend on how the node is created.

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If it is added from the ribbon (Windows users), contextual toolbar (macOS and Linux users), or context menu (all users), then the first physics interface of each type in the component is selected as the default.

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If it is added automatically when the physics interface is chosen in the or window, then the two participating physics interfaces are selected.

You can also select from either list to uncouple the node from a physics interface. If the physics interface is removed from the , for example is deleted, then the list defaults to as there is nothing to couple to.

	If a physics interface is deleted and then added to the model again, then in order to re-establish the coupling, you need to choose the physics interface again from the lists. This is applicable to all multiphysics coupling nodes that would normally default to the once present physics interface. See Multiphysics Modeling Workflow in the COMSOL Multiphysics Reference Manual.

Select pairs for which this coupling is to be used. The pairs must be defined under in the component.

In the case when the coupling is of the fixed geometry type, you can select which aspect of the interaction you want to model. From the list, select , , or .

In the case , pressure and viscous forces in the fluid affect the structure, but the structure does not affect the fluid flow.

In the case , the structural velocity acts as boundary condition for the fluid, but the fluid does not affect the structure.